Arming the Syrian rebels would be disastrous because Britain would be “pressing weapons into the hands of maniacs”, says Boris Johnson.

The Mayor of London warns David Cameron that the UK must not use Syria as an “arena for muscle-flexing” and says that any weapons sent to the country’s opposition could end up in the hands of al-Qaeda.

His comments come after several leading figures opposed any move by Mr Cameron to join President Barack Obama in providing greater assistance to the forces fighting Bashar al-Assad, the Syrian leader.

President Vladimir Putin of Russia, whose nation provides arms to the Assad regime, criticised the Prime Minister for considering arming rebels who “eat the organs” of their enemies.

The Deputy Prime Minister Nick Clegg also cautioned Mr Cameron against arming the Free Syrian Army, saying that if it were a good idea, Britain would have done it already. The former head of the Army, Lord Dannatt, said he feared any such assistance would lead Britain into further intervention, while the Archbishop of York, Dr John Sentamu, urged Mr Cameron to “tread very warily”.

But it is Mr Johnson’s comments that will do most to undermine Mr Cameron’s position on the issue.

Writing for The Daily Telegraph, Mr Johnson says the only solution in Syria is a “total ceasefire” and claims that it will be “impossible” to arm the rebels without weapons ending up in the hands of “al-Qaeda-affiliated thugs”.

“This is the moment for a total ceasefire, an end to the madness,” Mr Johnson writes. “It is time for the US, Russia, the EU, Turkey, Iran, Saudi and all the players to convene an intergovernmental conference to try to halt the carnage. We can’t use Syria as an arena for geopolitical point-scoring or muscle-flexing, and we won’t get a ceasefire by pressing weapons into the hands of maniacs.”

The crisis in Syria is threatening to overshadow the G8 Summit in Northern Ireland which begins on Monday.

Mr Cameron clashed with the Russian president at a Downing Street press conference on Sunday.

Asked by reporters whether he had “blood on his hands” for arming the Assad regime, Mr Putin said that his nation had acted in accordance with international law by delivering arms to the Syrian government.

He added: “I believe you will not deny the fact that one should hardly back those who kill their enemies and eat their organs – all that is filmed. Do you want to support these people? Do you want to supply arms to these people?”

Mr Cameron faces growing political opposition at home amid suggestions that he is in favour of joining the Americans in helping to assist rebels. He has been warned that he could be defeated in the Commons if he tries to win a parliamentary agreement for Britain to arm the rebels.

Mr Clegg insisted that the Government will not arm the rebels because it is not the right thing to do at the moment. “We’ve taken no decision to provide lethal assistance so we clearly don’t think it is the right thing to do now, otherwise we would have decided to do it,” Mr Clegg told the BBC.

Lord Dannatt, former head of the Army, warned that supplying arms to the Syrian opposition could turn into a “much larger intervention”.

“It’s a very complex situation,” he said. “And I think there is a real danger of an arm going in a mangle.”

He said the risk of supplying small arms is that it “becomes the thin end of the wedge”.

The Archbishop of York, Dr John Sentamu, warned that arming the rebels could be a “naive position to take” because it is impossible to know who the “good guys” are.

Julian Lewis, a Tory MP, said it would be “suicidal” for Britain to hand arms to an opposition known to include extremist elements.